Method of manufacturing film resistors



Aug. 8, 1967 TEIZO MAEDA ET AL 3,334,396

METHOD OF MANUFACTURING FILM RESISI'ORS 2 Sheets-Sheet 1 Filed July 29, 1964 q 4zo z4eamw w 0 4 m 0 w B a d f U 6 0 7% W INVENTOR ATTORNEYS so a Z O wfl z I L Z M 4/ M in Aug. 8, 1967 TEIZO MAEDA ET AL 3,334,396

METHOD OF MANUFACTURING FILM RESISTORS Filed July 29, 1964 2 Sheets-Sheet f3 1 1 4 fi i /00 2'00 300 400 I Time (hours) Res/fiance var/Own %)00/70) INVENTORS ATTORNEYS United States Patent Ufifrce 3,334,396 METHOD OF MANUFACTURING FILM RESISTORS Teizo Maeda, Moriguchi-shi, and Yuzo Mizobuchi,

Kadoma-shi, Japan, assignors to Matsushita Electric Industrial C0., Ltd., Osaka, Japan, a corporation of Japan Filed July 29, 1964, Ser. No. 385,889 Claims. (Cl. 29-620) ABSTRACT OF THE DISCLOSURE A method of manufacturing synthetic film resistors comprising a ceramic base and resistance film formed thereon including the steps of mechanically forming a precisely dimensioned ceramic base and subjecting said formed base to a high temperature heat treatment to form a microscopically smooth surface thereon and forming a resistance film on the ceramic base subsequently to the above-mentioned heat treatment.

This invention relates to a method of manufacturing film resistors, such as metal-film resistors on metal-oxidefilm resistors, comprising a ceramic base and a resistive film formed on the ceramic base.

An object of the present invention is to provide a method of manufacturing a film resistor comprising a ceramic base with a dimensional precision, without fear of deteriorating the electrical characteristics of the resistor.

Another object of the invention is to provide a method of manufacturing a film resistor, comprisingtrimming or beveling down ,the ,end corners of the ceramic base for facilitating terminal-cap assemblage without fear of deteriorating the electrical characteristics of the resistor.

There are other objects and particularities of the present invention, which will be made obvious to persons skilled in the art from reading the following detailed description, with reference to the accompanying drawings, in which;

FIG. 1 shows a side elevation and an end elevation of a ceramic rod base for a film resistor according to the present invention;

FIG. 2 shows a side elevation and end elevation of a ceramic rod which is trimmed at the end corners;

FIG. 3 is a graph showing direct-current load life characteristics of ametal-oxide-film resistor according to the present invention compared with conventional metal oxide film resistors; and

FIG. 4 is a graph showing direct current load life characteristics of metal-oxide-film resistors each manufactured with a ceramic base ground and treated at a temperature together with that of a conventional metal oxide film with a nonground ceramic base.

For mass production of film resistors, such as metalfilm resistors on metal-oxide-film resistors, in general, their ceramic base should be high in dimensional precision with neither curving nor Warping. In practice, however, a sintered ceramic base does not always satisfy the above requirements. In order to obtain a satisfactory ceramic base, it has been the practice to grind the base by use of centerless grinders. Grinding by centerless grinders gives a ceramic base having an apparently smooth surface when measured with surface roughness meters. In microscopic observation, such surface is very rough, so that the thickness of metal films or metal oxide films thereon cannot be said to be uniform, but the films are very thin at certain portions, whereby electrical characteristics,

3,334,396 Patented Aug. 8, 1967 such as load-life characteristics, humidity characteristics, etc., are deteriorated, resulting in useless resistors.

On the other hand, in manufacturing film resistors, after a resistive film has been formed on a ceramic base, terminal caps are applied onto the ends of the resistors. When the base 1 is not trimmed at the end corners 2 as shown in FIG. 1, terminal caps are very hard to apply onto the ends of the resistors. In order to facilitate the application of terminal caps, it has been a practice that end corners 2 are trimmed or beveled down as shown in FIG. 2. Such trimming is usually effected by use of grinders, but such grinding work is not efiicient in operation, resulting in high cost. For highly eflicient operation, resulting in relatively low cost, ball-mill Working is known, 'but such a working involves mutual rubbing of ceramic bases, or rubbing a ceramic base with an abrasive agent, such as carborundu'm, etc., and consequently, the surfaces of the ceramic base are rendered very rough in microscopic observation, resulting in deterioration of the elect-rical characteristics, as in the afore-mentioned case of grinding by use of centerless grinders.

According to the present invention, the surface of a ceramic base rod or tube is ground by use of a centerless grinder for the purpose of eliminating any curving and warping and of obtaining a predetermined dimensional precision for the external diameter, and then the base is subjected to heat treatment at a high temperature of 800 to 1400 C. for several minutes to several hours.

When the ceramic base is of a planar plate form or other than a tube, the surface of the ceramic base is ground by a suitable grinder, such as a planar or surface grinder, for the purpose of obtaining a predetermined dirnensional precision, and then the ceramic base is subjected to heat treatment the same as in the case of the rod or tubular ceramic base.

Further, according to the present invention, when the end corners of a rod or tubular ceramic base have been trimmed by ball mill working, the ceramic base is subjected thereafter to the heat treatment as described above.

Example A ceramic base rod of 6.8 mm. in external diameter and 50.5 mm. in length was obtained after surface grinding by use of a centerless grinder, and then the rod was subjected to a high temperature treatment. g. of stannic chloride (SnCl -5H O) and 4.6 g. of antimony trichloride (SbCl were dissolved in 60 cc. of methanol, and the solution was applied by spraying it onto the ceramic base rod heated to 680 C. to form thereon a surface layer or film of metal-oxide resistive material consisting of SnO containing Sb as an impurity. Curve C in FIG. 3 shows the DC. load life characteristics of the film resistor manufactured as above-mentioned, with the ceramic base heat treated at 1100 C. for 30 minutes after grinding. The experiment was effected in air at 40 C. by 7-watt DC. power supplied for periods of 1.5 hours duration at intervals of 0.5 hour over a total period of 500 hours, i.e., for 250 cycles.

In FIG. 3, curve A is for a film resistor with a ceramic base rod which has not been ground, and curve B is for a resistor with a ceramic base rod which has been ground but not heat treated. The resistor with a ceramic base ground by a centerless grinder but not heat treated remarkably drops in resistance at the beginning of the load test, while the resistor according to the present invention is extremely stable comparably to the resistor with nonground, non-heat-treated ceramic base rod as seen from the corresponding characteristic curve C of FIG. 3.

Theoretical explanation on such phenomena is not certain at the present, but it can be said in general that the surface of a ceramic base consists of crystalline and non-crystalline (glass phase) phases. In a non-ground ceramic base non-crystalline phase surrounds crystalline phase providing a microscopically smooth surface. When such a surface has been ground for improving the dimensional precision by use of a centerless grinder, the surface is rendered very rough in microscopic observation. In fact, when viewed by an electronic microscope, many scratches and pin-hole-like unevenness are observed in the ground surface. When such a ceramic rod having a microscopically rough surface is heat treated at a high temperature, according to this invention, the non-crystalline phase would be fused and provide a smooth surface even in microscopic observation, which would contribute to the stable load-life characteristics.

The temperature of the heat treatment should be selected in accordance with the kind of ceramic base material used. For example, forsterite ceramic of 2MgO-Si0 formulation is best heat-treated at temperatures ranging from 1000 to 1200 C., but the best temperature tends to vary, depending upon the amount and kind of noncrystalline phase contained in the forsterite ceramic. Further, the best temperature for heat treatment is lowered when the particle size of the centerless grinder used is smaller. In any case, the best temperature for heat treatment is the temperature which is as high as possible within the temperature range in which the non-crystalline phase begins to fuse, but the ceramic base is not deformed.

FIG. 4 shows the DC. load life characteristics of various film resistors manufactured by the present method with ceramic bases heat treated at various temperatures. In the figure, curve A is for a film resistor with a ceramic base neither ground nor heat treated, while curve B is for a film resistor with a ceramic base ground by a centerless grinder but not heat treated. Curves C, D, E, F and G are for film resistors which were heat treated at 1100 C., 1000 C., 900 C., 800 C., and 700 C., respectively. It is seen that there is substantially no effect for a heat treatment at 700 C., but with an increase in treating temperature, the load-life characteristics are stabilized more and more, and with ceramic bases heat treatments at temperatures higher than 1000 C., the stability is comparable to that with a non-ground ceramic base.

Grinding by a centerless grinder has been referred to in the foregoing description, but it is to be noted that the same is applicable to the cases when a ceramic base plate is ground by a planar grinder or the like, and when the end corners of ceramic rod or tube are trimmed by ball milling, before the heat treatment.

What is claimed is:

1. An improved method for making film resistors comprising a ceramic base consisting of a crystalline phase and a glassy phase with a resistive film formed thereon comprising the steps of precisely shaping the ceramic base to a given form by grinding, subsequently heating said shaped ceramic base to a temperature wherein the surface of said shaped ceramic base is made microscopically smooth, and forming said resistive film thereon.

2. An improved method for making film resistors as defined in claim 1 wherein said ceramic base is shaped by centerless grinding.

3. An improved method for making film resistors according to claim 1 wherein said ceramic base is shaped by planar grinding.

4. An improved method for making film resistors as defined in claim 1 wherein said ceramic base comprises forsterite and in which said base is treated at a temperature of 900 C. to 1200 C. after shaping.

5. An improved method for making a film resistor comprising a ceramic base of cylindrical form consisting of crystalline phase and glassy phase and a resistive film formed thereon and terminal caps applied to both ends of said ceramic base comprising the steps of beveling both end surfaces of said cylindrical base by means of a ball mill, heating said beveled ceramic base to a high temperature whereby the surface of said beveled ceramic base is made microscopically smooth, and forming a resistive film thereon.

References Cited UNITED STATES PATENTS 2,489,409 11/1949 Green et al 338-275 3,095,636 7/ 1963 Ruckelshaus 29-155.7

JOHN F. CAMPBELL, Primary Examiner.

J. L. CLINE, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,334,396 August 8, 1967 Teizo Maeda et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the heading to the printed specification, after line insert Claims priority, application Japan, Sept. 6, 1963,

Signed and sealed this 29th day of October 1968.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. AN IMPROVED METHOD FOR MAKING FILM RESISTORS COMPRISING A CERAMIC BASE CONSISTING OF A CRYSTALLINE PHASE AND A GLASSY PHASE WITH A RESISTIVE FILM FORMED THEREON COMPRISING THE STEPS OF PRECISELY SHAPING THE CERAMIC BASE TO A GIVEN FORM BY GRINDING, SUBSEQUENTLY HEATING SAID SHAPED CERAMIC BASE TO A TEMPERATURE WHEREIN THE SURFACE OF SAID SHAPED CERAMIC BASE IS MADE MICROSCOPICALLY SMOOTH, AND FORMING SAID RESISTIVE FILM THEREON. 